John Morton, a vascular surgeon at Christchurch Hospital, responds. :-

Varicose veins in the legs are very common. In carrying blood from the lower limbs back
towards the heart, gravity has to be overcome. Normally this is achieved by the muscles in
the limbs providing a pumping action, and valves in the veins preventing blood refluxing back
down the veins when the muscles relax.

Varicose veins sometimes run in families. These families do not seem to have enough valves
in their veins. Later in life the valves can be destroyed by clotting in the veins. When the
valves are absent, or fail for whatever reason, the vein wall is subjected to high pressure which
causes it to dilate and become elongated - "bulging varicose veins".